Add <$testcase> widget (#7817)

* Initial Commit

* Add note to preview build

* Fix whitespace and indenting

Thanks @pmario

* Fix crash with unset $tiddler attribute on <$data> widget

Thanks @CodaCodr

* Don't duplicate "description" field in test cases

* Use different background colours for nested testcase widgets

* Extend the testcase widget to run tests

* Add testcases to control panel

* Add a view template body template to render testcase tiddlers

* Test edition should display testcases

* Whitespace fixes

* Make testcase tiddler tempalte link to itself

* Styling tweaks

* Docs improvements

* Styling tweaks

* Run the new tw5.com testcases in the test edition

* Update data widget to display its content in JSON

* Add testcase convenience procedure

* Clearer testcases for data widget, and docs tweaks

* Don't expect our intentionally failing test to pass

* Extend testcase default template so that the display format can be chosen

It is selected by setting the variable "displayFormat"

* DataWidget docs typo

* Fix data widget not refreshing

* Links in testcase output switch to the tab containing that tiddler

Thanks to @btheado for the suggestion

* Docs update for 648855e8a5

* Wording tweak

* Add support for narrative tiddlers in test cases

* Documentation improvements

* Cleanup comments

* Remove obsolete code comments

* Simplify template

* Docs update

* Rename $:/core/ui/testcases/DefaultTemplate/SourceTabs from $:/core/ui/testcases/DefaultTemplate/Source

* Use the view template body for failing tests

* Don't reference the geospatial plugin

* "Test case" should be two words

* Fix handling of currentTiddler variable

Fixes problem reported by @btheado in https://github.com/Jermolene/TiddlyWiki5/pull/7817#issuecomment-2103704468

* Prepare for merging

core/palettes/Vanilla.tid

@@ -95,6 +95,9 @@ table-footer-background: #a8a8a8
 table-header-background: #f0f0f0
 tag-background: #ec6
 tag-foreground: #ffffff
+testcase-accent-level-1: #84C5E6
+testcase-accent-level-2: #E3B740
+testcase-accent-level-3: #5FD564
 tiddler-background: <<colour background>>
 tiddler-border: <<colour background>>
 tiddler-controls-foreground-hover: #888888